As population grows, the strain on the world's fresh water supplies will increase. Factors such as a pleasant climate and mineral resources, job growth and rising incomes contribute to population growth. By 2025, about 2.7 billion people, nearly one-third of the projected population, will live in regions facing severe water scarcity. Many prosperous and fast growing regions—the American Southwest, Florida, Asia, the Middle East—have inadequate freshwater supplies. The water needs of municipalities, industry, and citizens must be met, even as the difficulty and cost of developing new water resources increases.
Desalination has become a more popular option in regions where there is abundant water that is unsuitable for use due to high salinity, and there are opportunities for desalination plants that utilize thermal, electrical or mechanical energy to separate the water from the salts. The choice of the desalination process depends on many factors including salinity levels in the raw water, quantities of water needed, and the form of available energy.
Reverse osmosis is generally accepted as the most economical and energy-efficient method for desalination of highly saline water. Modern reverse osmosis membranes have such high salt rejection that they are capable of producing potable water, <500 ppm salinity, from seawater (nominally 35,000 ppm salinity) in a single pass through the membrane. Furthermore, some modern reverse osmosis systems are capable of achieving up to 50% recovery of fresh water from seawater. With 50% recovery, the salinity of the concentrated brine increases to about 70,000 ppm. Disposal of such brines presents significant costs and challenges for the desalination industry, which result in longer start-up lead times and higher cost of water. Brine disposal to surface waters in the United States requires waste water permits that prevent construction in certain high-demand areas. There are three basic ways to deal with brines from seawater desalination—discharge to the sea, deep well injection, and zero liquid discharge systems. The discharge of brines back into the sea can affect the organisms in the discharge area.
Evaporation and electrodialysis, which are proven processes for seawater desalination, can make a brine of considerably higher concentration than can be recovered from reverse osmosis, but these processes consume more energy than reverse osmosis in seawater desalination.
One problem that is of concern in many desalination processes is the formation of an unwanted precipitate, such as calcium sulfate scale. Calcium sulfate is typically present in saline water and has a relatively low solubility in water. Thus, calcium sulfate is known to precipitate in reverse osmosis processes causing various problems. For example, in evaporation processes, the high temperature at the heat exchange surfaces causes local supersaturation due to reduced solubility of calcium sulfate at elevated temperatures, even when the bulk solution is not saturated. In processes utilizing reverse osmosis and nanofiltration (NF), conditions of supersaturation can exist at the membrane surface due to buildup of ion concentrations in the boundary layer. Brackish groundwater often has enough calcium and sulfate ions to limit the amount of fresh water that can be recovered by desalination.
Disposal of the concentrated brine from reverse osmosis plants is also a major concern. The presence of dissolved salts adds to the density of water. The specific gravity (at 20° C.) of seawater (3.5% salts) is about 1.0263, and the specific gravity of high-yield reverse osmosis reject (7.0% salts) is about 1.0528. If this dense reverse osmosis reject were to be injected directly into the sea, it may accumulate at the bottom with a possible adverse effect on bottom-dwelling organisms.
Other than return to the sea, the alternatives for disposal of brines from desalination plants are limited. Evaporation ponds are generally undesirable and expensive due to the cost of land. Moreover, they are useful only in climates where evaporation rates exceed rainfall. Deep well disposal is often used for hazardous wastes, and it has been used for desalination brines in Florida, but capital costs make the process prohibitive. Furthermore, the applicability of deep well injection for large desalination plants is questionable because of the sheer volume of the brine.
In some applications, brines from desalination plants may also contain various pollutants that should not be discharged, even in small concentrations. For example, arsenic, selenium, and the like are sometimes present in seawater, groundwater or surface water. Concentrations of even a few parts per million of selenium or arsenic, for instance, may be considered hazardous. For example, agricultural operations in the Central Valley of California have a significant problem of selenium in the drainage waters that prevent reuse of the water for irrigation or for other purposes. Thus, the above pollutants can also create limitations in the use of reverse osmosis processes.
Seawater has many valuable constituents, but their value can only be realized if they can be recovered economically. There are ways to recover many of these valuable seawater constituents, but the economics of the recovery are often dismal because of the low concentrations of those constituents, and due to interference by other constituents of seawater.
One valuable component of seawater is sodium chloride (NaCl). Japan, for example, has no natural salt deposits, and land is too expensive there to allow the use of evaporation ponds for salt manufacture. For several decades Japan has relied on electrodialysis to recover table salt from seawater. The seawater is filtered and pumped at low velocity in a single pass through the desalting compartments of very large electrodialysis stacks. The voltage applied across membranes and solution compartments forces Na+ ions through the cation permeable membrane on one side of the compartment and Cl− ions through the anion permeable membrane on the other side of the compartment. The Mg++ ions, second most abundant cations in seawater, also migrate in the electric field, but Mg++ passage through the cation permeable membrane is hindered by a special coating on the membrane surface. The passage of SO4= ions is hindered by a coating on the anion permeable membrane. Thus the purity of the NaCl in the brine recovered by electrodialysis is substantially higher than the purity of brine prepared by evaporation of raw seawater. After concentration to 20% by electrodialysis, the brine is evaporated to dryness with the byproduct heat from the power plant used to generate the electricity for the electrodialysis.
Seawater is also used as the feedstock for the production of magnesium and bromine compounds. A commercial method for recovering Mg++ is to add a base (usually lime) to seawater in order to precipitate Mg(OH)2. One disadvantage that the recovery of magnesium from seawater has in comparison with magnesium recovery from magnesite is the low concentration of magnesium in the seawater. If the magnesium content of the brine feed could be increased at a reasonable cost, the production costs for magnesium would be reduced. Accordingly, this would allow manufacturers using seawater as a feedstock to compete more effectively with magnesium producers who use magnesite. Moreover, this would help alleviate the environmental damage associated with magnesite mining operations as well as the generation of the large amount of carbon dioxide incident to the processing of magnesite.
In view of the above, a need currently exists for processes and systems that can efficiently recover purified water from saline water. In particular, a need exists for a process and system that is capable of handling brines from reverse osmosis processes in a manner so that the brine can be discharged safely or in a manner such that there is zero liquid discharge. A need also exists for a process and system for recovering valuable chemicals, such as sodium chloride, magnesium, bromine and the like from saline water. A need further exists for a process and system for removing pollutants from saline water and possibly converting the pollutants into usable resources.